A disc system such as an optical disc apparatus, for example, reproduces information recorded on an optical disc by irradiating the optical disc with laser light and detecting the intensity of its reflected light. In present optical disc apparatuses, in particular DVD (digital versatile disc) players and the like, it is required for one apparatus to support reproduction from as many types of discs as possible among a variety of discs such as DVD (single-layered), DVD (double-layered), DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, CD, CD-R and CD-RW. For this reason, at the time of startup of an optical disc system or change of a disc, it is necessary to identify which type of disc is mounted, optimize the system setting according to the identification result and start reproduction promptly.
DVDs and CDs have been standardized, and thus disc carriers should desirably be fabricated to conform to the standards. However, with a great number of media being available in the market, the reality is that discs nonconforming to the standards have been increasingly put in the market due to variations in fabrication process and the like. It is also noted that measures against defects such as flaws and fingerprints have increasingly become important.
To accommodate such variations in discs and variations in properties of pickups and the like, a signal reproducing device provided with an automatic gain control (AGC) function for a reproduction signal has been adopted (see Patent Literature 1, for example).
FIG. 10 is a block diagram of a conventional signal reproducing device of this type, in which an optical disc 901 (recording medium) has an information signal recorded along a helical or concentric track thereon,
a rotation drive section 902 rotates the optical disc 901,
a pickup 903 is an optical pickup that converges a light beam on an information recording surface of the optical disc 901 to form a light spot and detects the intensity of its reflected light to thereby output various types of information signals (reproduction signals),
a focus actuator 904 moves the pickup 903 in a direction vertical to the disc plane of the optical disc 901,
a tracking actuator 905 moves the pickup 903 in a radial direction of the optical disc 901,
a focus control section 906 controls the focus actuator 904 according to the output of the pickup 903,
a tracking control section 907 controls the tracking actuator 905 according to the output of the pickup 903,
a head amplifier 908 amplifies the reproduction signal outputted from the pickup 903,
a variable gain amplifier 909 adjusts the amplitude of the reproduction signal outputted from the head amplifier 908,
an amplitude detection section 910 detects the amplitude of the reproduction signal outputted from the variable gain amplifier 909,
a comparator 911 compares the amplitude detected by the amplitude detection section 910 with a predetermined amplitude, and
a gain control section 912 control the gain in the variable gain amplifier 909 according to the output of the comparator 911.
In the conventional signal reproducing device configured as described above, in disc reproduction, the focus control section 906 and the tracking control section 907 perform processing for stabilizing focus servo and tracking servo based on the output of the pickup 903, and the focus actuator 904 and the tracking actuator 905 are driven to execute focus servo and tracking servo. In this way, the pickup 903 is allowed to follow the track while maintaining the focused state.
The AGC using the variable gain amplifier will be described with reference to FIG. 11. FIG. 11 schematically shows waveforms in disc reproduction, in which FIG. 11(a) is a schematic view of a reproduction signal from the pickup 903 amplified by the head amplifier 908, and FIG. 11(b) is a schematic view of an output signal of the variable gain amplifier 909. The output of the variable gain amplifier 909 is inputted into the amplitude detection section 910, which then generates an amplitude signal. As described in the Patent Literature 1, the amplitude of a reproduction signal is normally detected by calculating the difference between the upper envelope and the lower envelope of the reproduction signal. The detected amplitude is then compared with a predetermined amplitude level by the comparator 911. Based on the output of the comparator 911, the gain in the variable gain amplifier 909 is determined by the gain control section 912. The gain in the variable gain amplifier 909 is thus controlled so that the output of the variable gain amplifier 909 has the predetermined amplitude. Meanwhile, measures have been taken for a reproduction signal from the actual disc surface to respond to various circumstances including the focus control state, the tracking control state and the state of the disc surface.
When reproduction is made from a flawed disc or a dirty disc such as a so-called fingerprinted disc, the amplitude of a reproduction signal frequently increases/decreases as shown in FIG. 11(a). With response of AGC, however, control is made to give a predetermined amplitude as shown in FIG. 11(b). More specifically, with response of AGC, even when the amplitude of the reproduction signal has greatly decreased as in the time period t1 to t2, the degree of the decrease is minimized. Also, when the amplitude gradually changes as in the time period t3 to t4, control is made to maintain the predetermined amplitude.
Patent Literature 1: Japanese Laid-Open Patent Publication No. 2000-134049